Japanese Patent Application Laid-open No. 11-241601 discloses an axial-flow gas turbine engine comprising stator vanes and rotor blades, wherein a cross section in the axial direction of an inner peripheral wall of a casing to which the stator vanes and the rotor blades are connected has a concave part that recedes radially inward relative to a straight line joining the front edge of the root of the stator vane on the front side and the rear edge of the root of the rotor blade on the rear side.
Furthermore, U.S. Pat. No. 5,466,123 discloses an arrangement in which a cross section orthogonal to the axial direction of an inner peripheral wall of a casing supporting stator vanes of a gas turbine engine has sinusoidally alternating concave parts and convex parts.
It should be noted here that when the thickness of a turbine airfoil of a gas turbine engine is decreased in order to reduce the weight of the airfoil without changing the material thereof, it is necessary to increase its stagger angle γ (see FIG. 1), but since the curvature of the front half on the upper face of the airfoil increases with an increase in the stagger angle γ, the flow rate on the upper face of the airfoil accelerates and decelerates rapidly and, in particular, in the case of a high load blade having a high rotor inlet relative Mach number, a shock wave is generated beyond a critical Mach number, and as a result there is the problem that the pressure loss increases.